You have the Nodraw texture on a brush which is a displacement. In Hammer are some buttons on the top right. One of them toggles between displacements and brushes (the displacements get their original shape) and one toggles the faces of the displacement brushes which are destroyed. I think one of the icons look like two "D"-letters. The other one is next to it. So if you activate both all displacements are brushes with all faces visible. Replace every Nodraw texture on it. Even the destroyed faces mustn't have the nodraw texture on it.